Oh yeah, I used to take advantage of the free admission days back when I had an Illinois ID. Crowded, yes, but free is free, and I was all but broke nonstop back in those days.
Typically, with homeschooled children, I can plan vacations to be during weekdays to avoid the most popular times (this helps in getting campsites at state or national parks, for example)—especially if my wife isn't going. However, I've found that it works out best for me at work if I do a Wednesday–Monday trip instead of a Monday–Saturday trip. And, in this particular case, my plan is to visit the Art Institute on a Sunday. So yeah, believe me, I'm already planning to reserve well in advance. There's no way I would expect to just show up at the Institute and expect a bunch of same-day timed tickets to still be available.